1. Field of the Invention
The present invention relates to an optical bi-directional transceiver module, and more particularly to an optical bi-directional transceiver module suitable for an optical transmission/reception operation on the condition that an interval between two wavelength bands such as C and L bands is very narrow.
2. Description of the Related Art
The competitiveness of the countries in the 21st century information society can be improved by not only the expansion of an optical communication infra-structure, but also easier access/usage by the general public.
The optical communication technology can be gradually developed to widely accommodate the extensibility of a communication network and the capacity for preparing the service for the variable demand. With the increasing development of the optical communication technology, an optical communication system is being designed in the form of an integration circuit capable of implementing a wavelength division multiplex (WDM) system at a high speed. As a result, the development of the optical communication technology is being required for a variety of technical fields.
The most favorable scheme from among a variety of methods of the basic structure of the passive optical network (PON) is an FTTx scheme based on ATM passive optical network (A-PON) proposed by a full service access network (FSAN) created in 1995. The FTTx scheme has been developed for the Gigabit Ethernet PON (GE-PON), and has been made commercially available.
In addition, a WDM-PON system for providing a large number of services and the ultimate access network service can provide subscribers or users with the highest-quality access network service due to a wavelength re-set function, and can configure a flexible network, such that many developers or companies are conducting intensive research into the WDM-PON system and associated technology.
The WDM-PON system arranges N optical transceivers such as N optical network units (ONUs) on an optical line terminal, and performs wavelength division of downlink and uplink signals using an optical router at the OLTs and a remote node in order to multiplex the downlink and uplink signals, such that the multiplexed signals are transmitted to a destination.
The WDM-PON network requires the low-priced single wavelength light-sources technology, such that it is the most important technology capable of implementing the WDM optical communication technology at the metro and access networks.
The optical module from among overall constituent elements of the system is the most difficult technology, such that it encounters the bottle-neck of a variety of developing processes, and greatly affects the costs of the system hardware.
A conventional Fabry-Perot laser system can be easily manufactured by a simple fabrication process, and has a high production yield, such that it is widely used for a low-speed access network. However, as the transfer rate becomes higher and the WDM-PON system is generalized, the 32-channel or 64-channel single wavelength laser system is required.
Presently, the DFB laser system equipped with a diffraction grating including a resonator has been widely used, however, it should be noted that the DFB laser system has a complicated manufacturing process and a low production yield. Therefore, many developers are conducting intensive research into a method for adapting several semiconductor lasers, each of which has a simple manufacturing process and the high-quality optical characteristics, to the PON system.
Since the uncooled-type DFB LD system can be manufactured with the relatively-low costs as stated above, it is suitable for a mass-manufacturing process, and the fabrication process and package can be made commercially available, such that it can be easily used by users.
However, the uncooled-type DFB LD abruptly changes unique wavelength and optical output characteristics according to temperature, its operation characteristics have the high dependence on temperature, and the costs of the uncooled-type DFB LD are relatively higher than those of the GE-PON system.
The cooled-type DFB LD can be easily manufactured by a simple process, such that it is suitable for a mass-manufacturing process, and has superior wavelength adjustment characteristics in the case of a narrow interval between channels. In addition, the cooled-type DFB LD has a complicated package process of the high costs, such that it is difficult to be applied to an access network.
The VCSEL does not require a fine arrangement process for an optical fiber, has low power-consumption characteristics, and can estimate wafer-level element characteristics. However, the VCSEL has difficulty in maintaining the above-mentioned characteristics, such that it is unable to be used as the light source of the WDM-PON system.
The DBR LD has superior wavelength variability and single wavelength characteristics. However, it has a complicated process and a complicated driving circuit such that it is manufactured with very expensive cost. As a result, the DBR LD is also unable to be used as the light source of the WDM-PON system.
The External Cavity Laser (ECL) has low optical output characteristics due to the mode transition phenomenon and the structural problem of an external resonator. However, the ECL has a low temperature-dependency and can easily adjust the wavelength without using the TEC, and can test the level of individual components. Also, the ECL is manufactured with low costs such that it can be suitable for the system such as the PON system. As a result, many developers are conducting intensive research into the ECL due to the above-mentioned advantages. However, indeed, the ECL is unable to be easily applied to the WDM-PON system, because it must satisfy the maintenance and operation encountered by the fixed wavelength of the ONU and must guarantee the optical transceiver module of a variety of wavelengths.
In the light of advantages and disadvantages of the above-mentioned light sources, it is difficult to search for the optimum solution for a low-priced optical transceiver module capable of being used for the PON system. Recently, an improved WDM-PON technology based on an injection mode locked FP LD proposed by Novera Corporation has been disclosed in the Korean Patent Laid-open Publication No. 2003-63085 issued on Jul. 28, 2003.
However, the conventional WDM-PON optical transceiver module has an optical transmitter and an optical receiver separated from each other. The optical transmitter and the optical receiver are coupled to each other by the fusion splice, and are finally mounted to a driving circuit and a receiving circuit, resulting in the occurrence of the complicated structure. As a result, due to the complicated structure, the increasing costs of the above-mentioned WDM-PON optical transceiver module are inevitable, and the WDM-PON optical transceiver module cannot be manufactured in the form of a small-sized product.
In order to solve the above-mentioned problems, the bi-directional module shown in FIG. 1 has been proposed by the WDM-PON system.
FIGS. 1˜3 are conceptual diagrams illustrating a conventional optical bi-directional transceiver module.
The bi-directional transceiver includes an optical transmitter 10 and an optical receiver 20 in a single housing 50. In more detail, the A-wavelength light signal and the B-wavelength light signal pass through the ferrule 40, are transmitted to the A/B band filter 30 having the slope of 45°, and are reflected. The transmitted light signal is incident on the optical transmitter 10, and the reflected light signal is incident on the optical receiver 20.
However, if the incident light is indicative of the C-band having the wavelength of 1530˜1560 nm or the L-band having the wavelength of 1570˜1600 nm, a difference between the C-band and the L-band is about 10 nm.
In the case of using the C/L band filter 30 of the slope 45°, the difference in reflection characteristics between S-polarization and P-polarization shown in FIG. 3 greatly occurs within the band, such that the transmission and reflection of the C/L band cannot be controlled. As a result, the bi-directional optical transceiver module has the problem of polarization.
If the S-polarization and the P-polarization are different from each other within the wavelength range of 1530˜1560 nm (or the wavelength range of 1570˜1600 nm) as shown in FIG. 3, the optical reflection characteristics of the corresponding band are distorted according to the polarization, such that it cannot be used as a normal filter.
Therefore, in the case of manufacturing the bi-directional optical transceiver module of FIGS. 1˜2, the normal filter characteristic can be acquired only when an interval between two bands (e.g., 1310 nm/1550 nm, C-band/S-band, or C-band/E-band) may be at least several tens of nanometers (nm).